1. Field of the Invention
This invention relates to liquid cooled fast flux nuclear reactor cores, and more particularly to a core of assemblies of hexagonally bundled fuel rods in an open-lattice array.
2. Description of the Prior Art
Proposed cores for liquid metal cooled fast nuclear reactors typically include closely packed hexagonal fuel assemblies whereby the core approaches the configuration of a right circular cylinder. Typical fuel assemblies include a plurality of fuel rods of circular cross section bundled in a triangular array within a full length thin walled duct. The fuel rods are supported at one end, allowing free axial expansion, and laterally supported along their length by wire wraps about the rods or by egg-crate type grid structures positioned at selected locations along the assembly length within the duct.
Such full-length ducted assemblies limit coolant cross flow among adjacent fuel assemblies, increase the amount of metal in the core, thereby lessening nuclear efficiency, and increase the pressure drop through the core which further decreases efficiency. These characteristics can be mitigated by use of open-lattice fuel assemblies which eliminate the full length duct, utilize full length support members, and position grid structures at selected axial positions, as well known in the generally square fuel assemblies of light water reactors having fuel rods positioned in a square array.
The fuel assemblies in light water reactors use supportive guide tubes disposed within the fuel rod array to carry axial loads and are designed to allow direct contact among grids of adjacent fuel assemblies. Such grid contact, however, is undesirable for a fast flux reactor. Swelling and distortion of components in the fast neutron flux can be greater than in the thermal neutron flux of a light water reactor. Such distortions, if not carefully controlled, could result in excessive contact and binding among adjacent assemblies severely complicating refueling operations. Additionally, contact among grids can result in coolant cross flow starvation at the point of contact, thereby increasing the localized temperature of the components and the likelihood of damage and binding.
Light water reactors, having fuel rods positioned on a square array, can readily utilize grids, the outer surfaces of which are generally flat. Thus, lateral displacement or load transfer among fuel assemblies is through the contact of two flat, parallel surfaces. And, the square rod pattern can be readily continued across assembly interfaces with all fuel rods laterally aligned although spaced a greater distance at the assembly interface. However, if continuity of a triangular fuel rod array is to be maintained across the interface of hexagonal fuel rod bundles, and excessive space between assemblies avoided, adjacent assemblies must be displaced from one another by one-half of the pitch of the fuel rod array. A very complex core structure, and core lateral restraint structure, would result if all of the hexagonal assemblies throughout a core were displaced from one another by one-half of the fuel rod pitch. Additionally, to avoid excessive spacing at the assembly interface, and to maintain a close approximation of the triangular rod array across adjacent assembly interfaces, nesting of fuel rods is desirable. Nesting, however, produces a large number of small interfaces among adjacent grids. Small lateral displacement can accordingly result in grid-to-grid contact which, as stated above, is undesirable.
It is thus desirable to provide an open lattice assembly for a liquid cooled, fast neutron flux reactor so as to achieve, among other advantages, coolant cross flow among assemblies. It is further desirable to provide a core of open lattice hexagonal assemblies which approximate maintenance of a triangular rod array across the interface of adjacent assemblies while maintaining a relatively simple total core arrangement and lateral support structure.